Boring Sponge

Thankfully for us softies, Boring Sponges don't burrow into flesh. That'd be too easy. They prefer the challenge offered by the tough shells of snails and bivalve molluscs, and the stony skeletons of corals.

Being sponges who are armed with neither hammer nor chisel, or indeed arms, they achieve their excavations by means of chemical magicks and potions of unmaking. In other words, they secrete chemicals that dissolve the calcareous material of shells and coral skeletons.

Some Boring Sponges construct elaborate tunnels and passageways as they grow, but you wouldn't see it because it's all hidden beneath the surface of their host. All you see on the outside is an array of pores with tubes poking out here and there. They need these tubes to take in and expel water because they still feed by filtering out bacteria, just like other sponges.

I guess it's nice of them to not eat their host, but they still cause a lot of trouble. The victim constantly attempts to make repairs but they simply don't stand a chance against the chemical potions of their unwanted guest.

Some Boring Sponges can also encrust their host's shell and eventually smother it. Either way, their victim eventually dies and the sponge may continue dissolving that shell until disappears entirely. Gone in a long, drawn out puff of calcium carbonate.

When you think about it, it's a horrible way to go! A fleshy mound of animal cells infiltrating your home, carving holes in your haven, weighing you down, engulfing you and destroying you. And after all that, it turns out an empty shell would've done just as well! Whether they conduct their endeavours on a living animal or an discarded shell appears to be a matter of total chance.

On the other hand, it's always sort of cool to discover a new and interesting way in which nature destroys lives, especially when something as unexpected and humble as a sponge is the one doing it.